docs: document Oracle 26ai vector store backend in README

[akuppala21]

Update the top-level README to reflect the partner/oracle branch: Oracle AI Database 26ai as a pluggable vector store with hybrid vector + Oracle Text retrieval and GPU-accelerated HNSW index builds via NVIDIA cuVS. Add an Oracle backend get-started runbook (Helm/OKE), correct the default LLM to Nemotron 3 Super 120B, and link the Oracle deployment docs.

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• I am familiar with the Contributing Guidelines.
• All commits are signed-off (git commit -s) and GPG signed (git commit -S).
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• If adjusting docker-compose.yaml environment variables have you ensured those are mimicked in the Helm values.yaml file.

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